Conventionally, in measurement of pressure or flow rate in a percutaneous transluminal coronary angioplasty (PCI), the fractional flow reserve (FFR) is used as an important diagnostic index. The PCI is a treatment method for ischemic heart disease. The method increases the blood flow rate by dilating the cardiac coronary artery stenosed such as by an atheroma (a deposit or plaque inside an arterial vessel that are formed of cells or dead cells containing fatty substances, calcium and various fibrillary connections). The FFR index indicates the degree of blood flow constriction due to a stenotic lesion and is expressed as a ratio of a blood flow rate at a portion distal to the stenotic lesion site to a normal blood flow rate there. Specifically, a pressure Pa in the aorta proximal to a stenotic lesion site and a pressure Pd in a coronary artery distal thereto are measured, to calculate the FFR index from Pd/Pa. This will be explained in a little more detail with reference to FIGS. 6A and 6B. FIG. 6A is a schematic diagram showing stenotic lesions produced in a blood vessel, wherein the arrows indicate the blood flow direction. FIG. 6B is a model diagram of a pressure change in the blood vessel corresponding to FIG. 6A. In FIG. 6B, the vertical axis represents a maximum pulse pressure in the blood vessel and the horizontal axis represents an optical fiber length Lof. The optical fiber length here is the distance from a given start point, which is defined as zero, of the optical fiber to the distal end of the sensor. Designating stenosis positions of a lesion site Pc1 and a lesion site Pc2 at S1 and S2, respectively, the pressures in the blood vessel (blood pressures) along these sites gradually decrease from a reference pressure P0 (corresponding to the above described pressure Pa) as shown in the figure. And expressing pressures gradually decreased at the stenosis positions S1 and S2 by P1 and P2, respectively, P1/P0 and P2/P0 are called the fractional flow reserves (FFRs) at the respective positions. When either value of the FFRs is less than 0.75, the above-described PCI is applied. Ordinarily, the pressure Pa is measured at the head of a guiding catheter and the pressure Pd is measured with a pressure sensor at the head of a dedicated catheter called a pressure wire. In order to diagnose a symptom such as due to not a single stenosis but multiple stenoses or a physiological state that is information after a stent is placed, blood pressure or velocity is demanded in the PCI to be measured not as one point value but as a distribution.
In more detail, taking into account the size of a heart valve and the longitudinal size of a coronary artery stenosis, resolution of about 3 mm to 5 mm is desired in measurement such as of a longitudinal pressure distribution. The finer the diameter (for example, less than 0.4 mm) of a measurement probe, the better for insertion into a small coronary artery and through a heart valve, and a probe suitable for the measurement needs to have an appropriate stiffness and an optical fiber supporting mechanism. Considering the above, it is difficult for a probe to satisfy a specification of 0.4 mm or less for the outer diameter when a plurality of optical fibers are used in order to achieve the present measurement purpose. Moreover, in order to be able to measure temperature, velocity and the like simultaneously in addition to pressure measurement of a diseased site and to perform the measurement without affecting heartbeat, a multifunctional sensor without using an electrosensor is requested.
Conventionally, there has been a fiber Bragg grating (FBG) sensor as an optical fiber sensor used for such a purpose. However, the sensor needs formation of an FBG in the fiber. The original function of the sensor is temperature measurement through stretch or thermal deformation of the optical fiber, and it is difficult to particularly measure only pressure itself. In addition to the proposal so far, a pressure conversion mechanism is necessarily provided at a section where the FBG senses pressure. Thus, spatially continuous pressure cannot be measured. Moreover, a plurality (three or more) of fibers are needed to satisfy a specification of multifunctionality (multi-measurement function) capable of measuring quantities other than pressure, thus posing a hurdle to meet requirement of finer diameter (see, for example, Patent Document 1). Furthermore, since there is no sensor-functional section between FBGs, it is essentially difficult to measure a spatially continuous signal (see, for example, Non-Patent Document 1).
Although a quick multipoint measurement of temperature and pressure and a measurement of multi-parameters is enabled by coating an FBG portion with Zn metal vapor or the like, the sensor sensitivity is insufficient. Furthermore, while shape change of a measurement fiber incorporated catheter used for medical purposes causes strain in the measurement fiber, the frequency change by the strain is larger than that by pressure, thus posing difficulty in distinguishing a pressure signal from a strain signal.
There has been a sensor system that employs a single probe to improve the above problems. The sensor system is for measuring pressure and a flow rate by use of four microelectromechanical optical sensors (MEMS). Since apertures for pressure measurement are necessarily formed in the probe surface at positions corresponding to attached positions of the sensors, the measurement is limited to only several points located at certain intervals, thus posing a major obstacle in actual use. Moreover, need of a plurality of optical sensors is disadvantageous for making the probe finer.
Furthermore, since the sensors have such a complicated structure as shown in FIG. 18 that a plurality of apertures are necessarily formed in the probe surface at pressure sensing positions (see the cross sections A1-A1, B1-B1, and C1-C1 in FIG. 18), use of the sensors includes a problems from a safety viewpoint. Still further, when an electrosensor, which is another kind of sensor, is concurrently used for multifunctionality, influence to the heart and lungs must be taken into consideration (see, for example, Patent Document 2).
Addition to the above, the conventional technology further raises the following general problems in measurement of pressure and the like in a blood vessel. The first point is that since the sensor is not for a distributed measurement but for a point measurement, the measurement points are finite and restricted by the number of sensors. The second point is that since the measurement needs a plurality of sensors and measurement points are limited to several points as described above, the blood vessel length where the pressure and the like can be measured without moving the probe is shorted. The third point is that since a plurality of sensors are needed, the probe cannot be formed to have a diameter finer than a certain level; and the measurement is difficult for a vascular stenosis having a plurality of lesion sites such as because the probe has a complicated structure. The fourth point is that use of a plurality of fibers having different sensitivities fundamentally involves an influence due to the variations in sensor sensitivity; and since the variations in sensor sensitivity needs all products of the sensors to be calibrated though the sensor are thrown away after used temporarily, the sensors do not lend themselves to mass production and wasteful. From these points, it is conceivable that conventional sensors are practically difficult to use as a sensor for measuring pressure and the like in a blood vessel.